1. Field of the Invention
The present invention relates to a method for annealing silicon thin films and polycrystalline silicon thin films prepared using the method. More particularly, the present invention relates to a method for annealing silicon thin films, which, in a substrate on which an insulation layer and a silicon film are subsequently formed, preheats the silicon film in a temperature range where the substrate is not transformed during the process in order to generate an intrinsic carrier therein so that a resistance is lowered to a value enabling Joule heating, applies an electric field to the preheated silicon film in order to induce Joule heating by means of movement of the carrier, resulting in crystallization, removal of crystal lattice defects, and crystal growth, and also relates to polycrystalline silicon thin films of good quality, prepared using the method.
2. Description of the Related Art
Generally, amorphous silicon (a-Si) has some disadvantages such as low aperture and poor mobility of electrons acting as charge carriers, and it is also not in accordance with the CMOS procedure. Meanwhile, a polycrystalline silicon (Poly-Si) thin film element is capable of configuring a driving circuit required for writing an image signal to a pixel on a substrate like a pixel TFT-array, which was impossible with a-Si TFT. Thus, Poly-Si thin film element does not require connection between a plurality of terminals and a driver IC, thereby improving productivity and reliability and reducing thickness of a panel. In addition, the poly-Si TFT process may form micro structures on wires or the like since it may use the micro structuring process of silicon LSI as it is. Thus, since there is no limitation in pitch on TAB mounting of the driver IC, which exists in a-Si TFT, pixel reduction is easy and many pixels may be implemented in a small angle of view. When compared with a thin film transistor using a-Si, a thin film transistor using Poly-Si in an active layer has excellent switching ability and enables miniaturization to be converted into CMOS since a channel location of the active layer is determined by self-matching. For such reasons, the Poly-Si thin film transistor is used as a pixel switching element of an active matrix-type flat panel display (e.g., an LCD or an organic EL) and is an influential element for a large screen display and for practically implementing a driver-mounted COG (Chip On Glass).
Such a Poly-Si TFT may be manufactured under either high temperature condition or low temperature condition. For the high temperature condition, an expensive material such as quartz should be used for the substrate, so the method is not suitable for enlarging a screen size. Thus, research is mainly focused on the method for mass-producing a-Si thin films into Poly-Si under a low temperature condition.
To make Poly-Si at low temperature, there are used SPC (Solid Phase Crystallization), MIC (Metal Induced Crystallization), MELC (Metal Induced Lateral Crystallization), ELC (Excimer Laser Crystallization) and so on.
SPC requires high crystallization temperature and long processing time, though it may obtain uniform crystal structure with inexpensive equipments. Thus, SPC cannot use a substrate such as a glass substrate which experiences heat distortion at low temperature, and it has low productivity. In SPC, crystallization is possible when a-Si thin film is annealed for 1 to 24 hours at 600 to 700° C. In addition, the Poly-Si produced by SPC has many crystal lattice defects in the formed crystal grain since it accompanies twinning-growth when being changed from an amorphous state to a crystalline solid state. Such factors cause decrease of mobility of holes and electrons of the produced Poly-Si TFT and increase of threshold voltage.
MIC has an advantage that crystallization is accomplished at a far lower temperature than the case of SPC since a-Si is contacted with a specific metal. The metal for MIC may be Ni, Pd, Ti, Al, Ag, Au, Co, Cu, Fe, Mn and so on, and these metals are reacted with a-Si to form eutectic phase or silicide phase, thereby promoting low temperature crystallization. However, in case MIC is applied to the actual process of Poly-Si TFT manufacture, it causes serious contamination to metals in a channel.
MILC is an application of MIC. MILC forms a gate electrode instead of deposition of metal on the channel, deposits a thin metal layer coating on source and drain thinly in the self-aligned structure to cause metal induced crystallization, and then induces side crystallization toward the channel. Ni and Pd are the most frequently used metals in MILC. The Poly-Si manufactured by MILC shows excellent crystallinity and high field effect mobility compared with that of SPC, but it disadvantageously shows a high leakage current feature. That is to say, the metal contamination problem is not completely solved, though it is decreased to some extent compared with MIC. Meanwhile, as an improvement of MILC, there is used FALC (Field Aided Lateral Crystallization). FALC gives faster crystallization rate and shows anisotropy of crystal orientation, but it also does not completely solve the metal contamination problem.
The aforementioned crystallization methods such as MIC, MILC and FALC are effective in the point that the crystallization temperature is lowered, but they have a common feature that crystallization is induced by metal. Thus, they are not free from the metal contamination problem. As revealed, in case Cu is used as the metal, a pollution level measured at the center of the channel is 2.1% in MIC, 0.3% in MILC, and 0.11% in FALC.
Meanwhile, ELC, recently developed, enables preparation of Poly-Si thin film on a glass substrate by a low temperature process together with solving the metal contamination problem. A-Si thin film deposited using LPCVD (Low Pressure Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition) has a very large absorption coefficient against an ultraviolet region (λ=308 nm) which corresponds to the wavelength of Excimer laser, so the a-Si thin film is apt to be easily melted at a suitable energy density. In case of crystallizing the a-Si thin film with an Excimer laser, melting and congelation are accompanied within a very short time. In this aspect, ELC is not a low temperature process in the strict sense. However, ELC may produce Poly-Si within an extremely short time (several ten nano-seconds) without damaging a substrate, since crystallization is accomplished by melting and congelation occurring very rapidly in a local melting area affected by Excimer laser. That is to say, if a laser is irradiated on a-Si of a preform composed of glass substrate/insulation layer/a-Si thin film, only the a-Si thin film is selectively heated, thereby being crystallized without damage of the glass substrate positioned below. In addition, in case of Poly-Si which is generated during phase shift from liquid to solid, it shows more stable crystal grain structure and shows remarkably reduced crystal defects in the crystal grain compared to Poly-Si generated through solid crystallization. Thus, Poly-Si manufactured using ELC is superior to any other product of other crystallization methods.
In spite of that, ELC has several serious drawbacks. For example, there is a problem of a laser system in the radiant heat of a laser beam itself, a problem of a laser process that a process region is extremely limited, and a problem that a shot scratch remains on a large-size substrate. Such factors cause irregularity of crystal grain size of the Poly-Si thin film which composes an active layer of Poly-Si TFT. In addition, since Poly-Si generated together with a phase shift from liquid to solid accompanies volume expansion, serious protrusions form toward a surface from a point where crystal boundaries are made. This phenomenon has direct influence on the gate insulation layer which is a post-process, and particularly exerts a serious effect on the reliability such as hot carrier stress caused by irregular morphology of the interface between Poly-Si and the gate insulation layer.
In recent, SLS (Sequential Lateral Solidification) is developed to solve instability of the aforementioned ELC, and successfully stabilizes a process region of a laser energy density. However, it still does not solve the problems of shot scratch and protrusions. In addition, seeing the current trend that the flat display industry is growing quickly, the technique using laser to control a crystallization process of a substrate with a size more than 1 m×1 m which is to be mass-produced eventually still has problems. Furthermore, ELC and SLS has another problem that great investment is required initially and maintenance cost are high since they require very expensive equipments.